Definition QA is a set of activities for ensuring quality in the processes by which products are developed or services are delivered. QC is a set of activities for ensuring quality in products or services. The activities focus on identifying defects in the actual products produced or services delivered.
Focus on QA aims to prevent defects with a focus on the process used to make the product or deliver the service. It is a proactive quality process. QC aims to identify (and correct) defects in the finished product or service delivered. Quality control, therefore, is a reactive process.
Goal The goal of QA is to improve development and test processes so that defects do not arise when the product is being developed or the service is being delivered. The goal of QC is to identify defects after a product is developed and before it is released, or after a service is delivered.
How Establish a work process or procedure and the assessment of its adequacy. Periodic conformance audits of the system operation. Finding and eliminating sources of quality problems through tools and equipment so that customer's requirements are continually met.
What Prevention of quality problems through planned and systematic activities including listening to the involved players. The activities or techniques used to achieve and maintain the product quality, process and service.
Responsibility Everyone on the team involved in developing the product or delivering the service is responsible for quality assurance. Quality control is usually the responsibility of a specific team that tests the product for defects, or assess the service for problems.
Example Verification is an example of QA Validation/Software Testing is an example of QC
Statistical Techniques Statistical Tools & Techniques can be applied in both QA & QC. When they are applied to processes (process inputs & operational parameters), they are called Statistical Process Control (SPC); and it becomes part of QA. When statistical tools & techniques are applied to finished products or services (process outputs), they are called as Statistical Quality Control (SQC) and comes under QC.
As a tool QA is a managerial tool QC is a corrective tool

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References: Wikipedia, Difen, ISD